The field of the invention is nanoporous polymers.
Decreasing size and increasing density of functional elements in integrated circuits has generated a continuous demand for insulating materials with reduced dielectric constants. Among other approaches, inclusion of air into an insulating material has been successfully used to reduce the dielectric constant of the material, and various methods of introducing air into materials are known in the art.
In one method, a thermolabile component is incorporated into a polymeric material, and after curing the polymeric material, the thermolabile component is destroyed by heating. For example, Hedrick et al. describe in U.S. Pat. No. 5,776,990 blending of a thermostable polymer with a thermolabile (thermally decomposable) polymer. The blended mixture is subsequently crosslinked and the thermolabile portion thermolyzed. Blending a thermostable and a thermolabile polymer is conceptually simple, and allows relatively good control over the amount of porosity in the final polymer. However, positional control of the voids is generally difficult to achieve, and additional problems may arise where control over homogeneity and size of the voids is desirable.
In order to circumvent at least some of the problems associated with void size and distribution, the thermolabile portion can be grafted onto the polymeric strands. For example, block copolymers may be synthesized with alternating thermolabile blocks and thermostable blocks. The block copolymer is then heated to thermolyze the thermolabile blocks. Alternatively, thermostable blocks and thermostable blocks carrying thermolabile portions can be mixed and polymerized to yield a copolymer. The copolymer is subsequently heated to thermolyze the thermolabile blocks. While incorporation of a thermolabile portion generally improves control over pore size and distribution, the synthesis of such polymers is frequently challenging.
Regardless of the approach used to introduce the voids via thermolabile portions in a polymer mixture, structural problems are frequently encountered in fabricating nanoporous materials. Among other things, the porous polymer tends to collapse at the temperature at which the thermolabile component is thermolyzed. Moreover, since the voids are not formed by a mechanically stable structure, the porous polymers tend to collapse when the overall porosity exceeds a critical extent of about 30%.
In another method, structurally more stable void carriers are incorporated into the polymeric material. For example, Yokouchi et al. teach in U.S. Pat. No. 5,593,526 a process for producing a wiring board in which hollow or porous glass spheres are covered with a ceramic coating layer, and wherein the coated glass spheres are then mixed with a glass matrix. Yokouchi""s glass spheres help to reduce the dielectric constant of the wiring board, however, require coating by relatively cumbersome and expensive methods such as chemical vapor deposition, etc. Moreover, in order to create a stable structure between the glass matrix and the coated spheres, the mixture has to be baked at temperatures of about 1000xc2x0 C., which is unacceptable for most, if not all integrated circuits.
Alternatively, Sato et al. describe in U.S. Pat. No. 5,194,459 an insulating material that is formed from a network of hollow gas filled microspheres entrapped in a cured crosslinked fluorinated polymer network. Sato""s materials dramatically reduce the temperature requirements as compared to Yokouchi""s materials. Furthermore, Sato""s materials can be coated onto appropriate materials in a relatively thin layer while retaining tensile strength. However, all of Sato""s polymers include fluorine, which tends to reduce adhesion of the polymer to the materials employed in the fabrication of integrated circuits. Moreover, fluorine is known to cause corrosion of metal conductor lines. Still further, since the glass spheres in Sato""s polymer network are not covalently bound to the surrounding network, the mechanical integrity of the porous polymer composition may be less than desirable under certain conditions.
Although there are many methods of introducing air in a nanoporous material known in the art, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need to provide improved methods and compositions for nanoporous low dielectric constant materials.
The present invention is directed to methods and compositions for nanoporous polymers in which a set of first polymeric strands are crosslinked with each other to form a hollow structure, and in which a set of second polymeric strands are crosslinked with each other and coupled to the first set of polymeric strands via a covalent bond to form a nanoporous polymer.
In one aspect of the inventive subject matter, at least some of the first polymeric strands comprise an aromatic portion, and are preferably a a poly(arylene) and/or a poly(arylene ether). Particularly contemplated poly(arylene ethers) further comprise a triple bond and/or a diene. While the hollow structure may have various shapes, it is preferred that the hollow structure has a spherical shape that is no more than 10 nanometers, and more preferable no more than 3 nanometers in the largest dimension.
In another aspect of the inventive subject matter, the first polymeric strands are crosslinked with each other via a cyclic structure, and in a further preferred aspect, the first polymeric strand and the second polymeric strand are coupled together via a cyclic structure. Although not limiting to the inventive subject matter, it is preferred that the first and second strand belong to the same chemical class. In particularly contemplated nanoporous polymers, the first polymeric strand has a triple bond and the second polymeric strand has a diene, and the first and second polymeric strands are coupled to each other by reacting the triple bond with the diene.
In a further aspect of the inventive subject matter, the nanoporous polymer has a dielectric constant k, and it is generally contemplated that the nanoporous polymers have a dielectric constant k of no more than 2.5, and preferably no more than 2.1. With respect to the glass transition temperature Tg of contemplated nanoporous polymers, preferred polymers have a Tg of no less than 400xc2x0 C.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawing.